1. Technical Field
The invention relates to Group II-VI semiconductor lasers. In particular, the invention relates to a Group II-VI semiconductor laser based on MgZnSSe, CdZnSSe, and MgCdZnSSe having excellent characteristics, a low threshold voltage, and a low operating voltage, and to a method for manufacturing such a laser.
2. Description of the Prior Art
In recent years, solid-source Molecular-Beam Epitaxy (MBE), Metal-Organic Vapor-Phase Epitaxy (MOVPE), and gas-source MBE methods have been developed and used to form crystals when fabricating Group II-VI semiconductor lasers.
In a solid-source MBE apparatus, a substrate is attached to a holder mounted roughly in the middle of a chamber. A pump having a high pumping speed, such as an ion pump, is used to establish and maintain an ultrahigh vacuum within the chamber. The elements and dopants that are needed for crystal growth are then deposited on the substrate. The elements and dopants are emitted in the form of beams from respective cells, i.e. heating furnaces, and form successive, multiple layers on the substrate as a result of various physical and chemical processes. Closing and opening a shutter installed near the emission port of each cell makes it possible to facilitate the formation of super lattices and to control the layer thickness of a hetero-laser at the atomic level. The adsorption conditions under which the elements and dopants are deposited to form the multilayer structure on the substrate are determined by such factors as the substrate temperature, the surface condition of the substrate, and the type of element or dopant.
When crystals are grown by solid-source MBE, various compounds or mixed crystals can be formed without restriction in terms of substrate crystals, and the supply of the crystal structure elements or doping impurities can be accurately controlled.
In gas-source MBE, several of the solid sources used in solid-source MBE are replaced by gas sources. In both processes, crystal growth can be monitored by using, for example, various electron-beam diffracting devices, scanning electron microscopes, and analyzers.
The MOMBE method is a crystal growth method that uses metal-organic materials in an ultrahigh vacuum chamber, where solid sources or gas sources used in an MBE method are replaced with metal-organic materials. As a general rule, such metal-organic materials are also emitted in gas form or after they are liquefied or gasified. For this reason, gas-source MBE and MOMBE are collectively referred to herein as "gas-source MBE."
Gas-source MBE is similar to solid-source MBE in that it can yield multi-element mixed crystals and super lattices, but because gas-source MBE involves the incorporation of gas molecules or metal-organic molecules into the substance that is emitted, the cell structure and adsorption conditions, e.g. temperature and pressure inside the chamber, are somewhat different from those used in solid-source MBE.
MOVPE, mentioned above, reacts metal-organic materials at high temperatures, and thus allows epitaxial growth of several types of compounds and their mixed crystals.
Solid-source MBE can produce Group II-VI semiconductor lasers having a highly accurately controlled layer structure. This method, however, produces devices having high threshold voltages Vth (usually 15 to 30 volts), thus increasing the resistance between the device electrodes. This increase in resistance is attributed to the large Schottky barriers formed between the p-type cap layer and the electrodes. The result is excessive device power consumption, the generation of excessive heat which exacerbates the need for heat sink capacity, and the ensuing increase in device size and reduction in device life time.
While gas-source MBE can produce a device having a low threshold voltage Vth (usually 2.5 to 7 volts), gas-source MBE is disadvantageous in terms of crystal quality, and so has not yet been put to practical use.
Gas-source MBE and solid-source MBE resemble each other in that they both require an ultrahigh vacuum and have other similar features, such that the same apparatus can be used to practice both methods. When, however, a semiconductor laser is manufactured using gas-source MBE, the ambient gas or gas source remains in the chamber. This is one reason why the combined use of gas-source MBE and solid-source MBE has not yet been implemented in the fabrication of semiconductor lasers.